/*
 * Copyright 2014-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import axios from './axios';
import {Observable} from './rxjs';

export default (url, interval, initialSize = 300 * 1024) => {
  let range = `bytes=-${initialSize}`;
  let size = 0;

  return Observable.timer(0, interval)
    .concatMap(() =>
      axios.get(url, {
        headers: {
          range
        }
      }))
    .concatMap(response => {
      const initial = size === 0;
      const contentLength = response.data.length;
      if (response.status === 200) {
        if (!initial) {
          throw 'Expected 206 - Partial Content on subsequent requests.';
        }
        size = contentLength;
      } else if (response.status === 206) {
        size = parseInt(response.headers['content-range'].split('/')[1]);
      } else {
        throw 'Unexpected response status: ' + response.status;
      }

      // Reload the last byte to avoid a 416: Range unsatisfiable.
      // If the response has length = 1 the file hasn't beent changed.
      // If the response status is 416 the logfile has been truncated.
      range = `bytes=${size - 1}-`;

      let addendum = null;
      let skipped = 0;

      if (initial) {
        if (contentLength >= size) {
          addendum = response.data;
        } else {
          // In case of a partial response find the first line break.
          addendum = response.data.substring(response.data.indexOf('\n') + 1);
          skipped = size - addendum.length;
        }
      } else if (response.data.length > 1) {
        // Remove the first byte which has been part of the previos response.
        addendum = response.data.substring(1);
      }

      return addendum ? Observable.of({
        totalBytes: size,
        skipped,
        addendum
      }) : Observable.empty();
    });
}
